Hollande, however, has decided against going to law despite an initial threat to do so. And it is unclear whether the official "first lady of France", Valérie Trierweiler, will take legal action. She remains in hospital.

Gayet's action could end up with a decision applying European human rights legislation. If so, what would be the likely outcome?

It involves a case following the publication of book in 2007 by Susan Ruusunen, a former girlfriend of the then Finnish prime minister, Matti Vanhanen. She and her publisher were both prosecuted under Finland's privacy laws and initially acquitted. But, on appeal, the verdict was overturned.

Finland's supreme court then upheld the appeal court's verdict, though it narrowed the grounds on which it upheld the conviction:

"The only references which, according to the court, had illegally disclosed information about the prime minister's private life were the information and hints about the sex life and intimate events between the girlfriend and the prime minister… descriptions of their brief and passionate intimate moments as well as giving massages to each other, and accounts of their sexual intercourse."

Lustig then ploughed through the ruling on the case by the European court of human rights to discover what its judges thought about it. You can read it in detail in the original or look at Lustig's blog for a précis. It's his conclusion that is relevant to the Hollande affair:

"If my reading of the Strasbourg ruling is right, they're perfectly ok under European law to disclose his affair, because there is a justified public interest in how a political leader conducts himself in private.

(And if there were any doubts, his deeply embarrassing inability to answer a question about who is France's current First Lady should surely have dispelled them.)

But, according to the Strasbourg judges, what happens between the sheets should stay between the sheets. Once the bedroom door is closed, it stays closed - even for presidents and prime ministers."